Cutter fracture detecting system

ABSTRACT

A cutter fracture detecting system capable of easily detecting a fracture of a cutter having a plurality of cutting edges even when one of the cutting edges is broken. The cutter fracture detecting system has an observer for calculating a disturbance load torque of a spindle motor from a torque command value and a velocity feedback value, a band rejection filter for removing a given frequency component of the disturbance load torque which is determined by the number of cutting edges of the cutter and the rotational speed of the cutter, and a comparator for comparing an output signal from the band rejection filter with a predetermined reference value, and outputting a signal indicative of a fracture of the cutter when the output signal exceeds the predetermined reference value. In the event of a fracture of the cutter with plural cutting edges, a signal is generated that cannot be removed by the band rejection filter, and such a signal is monitored by the comparator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cutter fracture detecting system fordetecting a fracture of a cutter, and more particularly to a cutterfracture detecting system for detecting a fracture of a cutter which hasa plurality of cutting edges.

2. Description of the Related Art

Unmanned operation of a numerically controlled machine tool for a longperiod of time requires that any fracture of the cutter be detectedquickly for a cutter change.

To meet such a requirement, it has been customary to measure the loadtorque of a spindle, i.e., a drive current of a spindle motor, at alltimes, generate an alarm when the drive current reaches or exceeds acertain value, stop the machining process, and change the cutter.

However, in the case of a cutter having a plurality of cutting edgessuch as a milling cutter, the load current of the spindle motor remainssubstantially unchanged even if one of the cutting edges is broken, andhence it is difficult to detect such a cutter fracture.

SUMMARY OF THE INVENTION

In view of the aforesaid problems, it is an object of the presentinvention to provide a cutter fracture detecting system which is capableof easily detecting a fracture of a cutter having a plurality of cuttingedges even when one of the cutting edges is broken.

To achieve the above object, there is provided in accordance with thepresent invention a cutter fracture detecting system for detecting afracture of a cutter which has a plurality of cutting edges, comprisingan observer for calculating a disturbance load torque of a spindle motorfrom a torque command value and a velocity feedback value, a bandrejection filter for removing a given frequency component of thedisturbance load torque which is determined by the number of cuttingedges of the cutter and the rotational speed of the cutter, and acomparator for comparing an output signal from the band rejection filterwith a predetermined reference value, and outputting a signal indicativeof a fracture of the cutter when the output signal exceeds thepredetermined reference value.

The observer calculates a disturbance load torque free of accelerationand deceleration components from a torque command value and a velocityfeedback value. The disturbance load torque generally contains a certainfrequency component which is determined by the number of cutting edgesof the cutter and the rotational speed of the cutter, and such afrequency component can usually be removed by the band rejection filter.In a normal condition, the output signal from the band rejection filtercontains a very little frequency component. When even one of the cuttingedges of the cutter is broken, its frequency component is varied andfalls outside of the rejection band of the band rejection filter. Sincethe frequency component cannot be removed by the band rejection filter,the band rejection filter produces an output signal of increased level.When the output signal from the band rejection filter is compared with apredetermined reference value, therefore, the fracture of the cuttingedge of the cutter can be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a cutter fracture detecting systemaccording to the present invention;

FIG. 2 is a block diagram of a hardware arrangement of a numericalcontrol apparatus for implementing a drilling system according to thepresent invention;

FIG. 3 is a block diagram of an observer for estimating a disturbantload torque;

FIG. 4 is a diagram showing frequency characteristics of a bandrejection filter;

FIG. 5 is a diagram showing, by way of example, a filter input signalwhen a cutter is in a normal condition;

FIG. 6 is a diagram showing, by way of example, frequency components ofa signal inputted to the filter when the cutter is in a normalcondition;

FIG. 7 is a diagram showing, by way of example, a filter output signalwhen the cutter is in a normal condition;

FIG. 8 is a diagram showing, by way of example, a filter input signalwhen the cutter is in a fractured condition;

FIG. 9 is a diagram showing, by way of example, frequency components ofa signal inputted to the filter when the cutter is in a fracturedcondition; and

FIG. 10 is a diagram showing, by way of example, a filter output signalwhen the cutter is in a fractured condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will hereinafter be describedbelow with reference to the drawings.

FIG. 2 shows in block form a hardware arrangement of a computerizednumerical control (CNC) apparatus which implements a drilling systemaccording to the present invention. As shown in FIG. 2, the computerizednumerical control apparatus is indicated by the reference numeral 10.The computerized numerical control apparatus 10 has a central processor(CPU) 11 for controlling the computerized numerical control apparatus 10in its entirety. The central processor 11 reads a system program storedin a read-only memory (ROM) 12 through a bus 21, and executes controlover the computerized numerical control apparatus 10 in its entiretyaccording to the system program. A random-access memory (RAM) 13 storestemporary calculated data and display data. The random-access memory 13comprises a DRAM. A nonvolatile memory (CMOS) 14 stores a machiningprogram and various parameters. The nonvolatile memory 14 is backed upby a battery (not shown), so that the stored data will be retained inthe event of a power failure of the computerized numerical controlapparatus 10.

An interface 15 serves to interface the computerized numerical controlapparatus 10 with an external device 31 which may be a tape reader, atape puncher, or a tape reader/puncher. A machining program can be readfrom the external device 31 which comprises a tape reader through theinterface 15, and a machining program edited in the computerizednumerical control apparatus 10 can be outputted to the external device31 which comprises a tape puncher through the interface 15.

A programmable machine controller (PMC) 16 is provided in thecomputerized numerical control apparatus 10 for controlling a machinetool according to a sequence program which is generated in ladder form.Specifically, the programmable machine controller 16 converts an Mfunction, an S function, and a T function which are commanded by themachining program, into signals required by the machine tool accordingto the sequence program, and outputs the converted signals from aninput/output (I/O) unit 17. The outputted signals are supplied toenergize electromagnets on the machine tool and operates hydraulicvalves, pneumatic valves, and electric actuators. The programmablemachine controller 16 also processes signals from limit switches on themachine tool and switches on a machine control console (not shown), anddelivers the processed signals to the processor 11.

A graphic control circuit 18 converts digital data including the presentpositions of the axes, alarms, parameters, and image data into imagesignals, which are sent to a display device 26 of a CRT/MDI (Cathode-RayTube/Manual Data Input) unit 25, and displayed on the display device 26.An interface 19 receives data from a keyboard 27 in the CRT/MDI unit 25,and delivers the received data to the processor 11.

An interface 20 is connected to a manual pulse generator 32 forreceiving pulses generated by the manual pulse generator 32. The manualpulse generator 32 is mounted on the machine control console formanually positioning mechanical operable parts accurately.

Axis control circuits 41 ˜43 receive motion commands for the respectiveaxes from the processor 11, and outputs commands for the respective axesto servoamplifiers 51 ˜53, respectively. In response to the commands,the servoamplifiers 51 ˜ 53 energize respective servomotors 61 ˜63 forthe respective axes. The servomotor 63 which controls the feeding of aZ-axis rotates a ball screw 64 to control the position and feedingvelocity of a spindle head 74 connected to a spindle motor 73 in thedirection of the Z-axis. The servomotor 63 has a built-in pulse coder631 for detecting the position of the spindle head 74. A positionalsignal from the pulse coder 631 is fed as a pulse train back to the axiscontrol circuit 43. Although not shown, the servomotors 61, 62 whichcontrol the feeding of X-and Y-axes, respectively, have respectivebuilt-in pulse coders for positional detection. These pulse coders alsofeed positional signals as pulse trains back to the axis controlcircuits 41, 42. Linear scales may be used as such position detectors.

A spindle control circuit 71 receives a spindle rotation command and aspindle orientation command, and outputs a spindle velocity signal to aspindle amplifier 72. In response to the spindle velocity signal, thespindle amplifier 72 energizes the spindle motor 73 to rotate at arotational velocity commanded by the spindle rotation command. Thespindle amplifier 72 also positions the spindle at a position indicatedby the spindle orientation command.

A position coder 82 is coupled to the spindle motor 73 through gears ora belt. The position coder 82 rotates in synchronism with the spindlemotor 73, outputs feedback pulses through an interface 81 to theprocessor 11 which reads the feedback pulses. The feedback pulses serveto move the other axes synchronously with the spindle motor 73 to makeit possible to effect machining such as drilling. The feedback pulsesmay be converted into a velocity signal X1s by way of F/V(frequency-to-velocity) conversion.

The spindle control circuit 71 has a processor (not shown) for executingsoftware processing to perform functions, one of which is an observer710. The observer 710 estimates a disturbance load torque Ys acting onthe spindle motor 73 in response to the velocity signal X1s, etc. Thedisturbance load torque Ys is sent to the processor 11, which reads thedisturbance load torque Ys and carries out a predetermined process. Theobserver 710 and the process carried out by the processor 11 will bedescribed in detail later on.

A cutter having a plurality of cutting edges, e.g., a milling cutter 75,is mounted on the spindle head 74 of the spindle motor 73. The rotationof the milling cutter 75 is controlled by the spindle motor 73. Theposition and feeding velocity of the milling cutter 75 in the directionof the Z-axis are controlled by the servomotor 63 through the spindlehead 74.

The milling cutter 75 is fed and positioned in the Z-axis direction bythe servomotor 63. A workpiece 91 is fixedly mounted on a table 92 whichis controlled to move in the X-and Y-axis directions by the respectiveX-and Y-axis servomotors 61, 62 through mechanisms, not shown. Theworkpiece 91 is milled by the milling cutter 75 while being thuscontrolled to move.

The observer 710 for estimating a disturbance load torque will bedescribed below. A technical concept for obtaining a disturbance loadtorque with an observer has already been disclosed in Japanese laid-openpatent publication No. 3-196313 filed by the present applicant.

FIG. 3 shows in block form an observer for estimating a disturbance loadtorque. The disturbance load torque includes disturbance load torquessuch as a cutting load torque, a frictional torque of mechanisms, and soon, and is equal to all torques of the spindle motor except foracceleration and deceleration torques for accelerating and deceleratingthe spindle motor. The processing shown in the block diagram of FIG. 3is executed by the observer 710 of the spindle control circuit 71.

In FIG. 3, a current command value U1s is a torque command value whichis outputted to the spindle motor 73 in response to a spindle rotationcommand from the processor 11. The current command value U1s is appliedto an element 401 to energize the spindle motor 73. To an output torqueof the spindle motor 73, there is added a disturbance load torque X2 byan arithmetic element 402. An output signal from the arithmetic element402 is converted into a velocity signal X1s by an element 403 where Jrepresents the inertia of the spindle motor 73.

The current command value U1s is also applied to the observer 710. Theobserver 710 estimates a disturbance load torque from the currentcommand value U1s and the velocity X1s of the spindle motor 73. Velocitycontrol of the spindle motor 73 is omitted here, and only processingoperations for estimating a disturbance load torque will be describedbelow. The current command value U1s is multiplied by (Kt/J) by anelement 411, and then outputted to an arithmetic element 412. To anoutput signal from the arithmetic element 412, there is added a feedbacksignal from a proportional element 414 by an arithmetic element 412, andthen there is added a feedback signal from an integral element 415 by anarithmetic element 413. Output signals from the arithmetic elements 412,413 are expressed in a unit of acceleration. The output signal from thearithmetic element 413 is applied to an integral element 416, whichoutputs an estimated velocity XX1 for the spindle motor 73.

The difference between the estimated velocity XX1 and the actualvelocity X1s is determined by an arithmetic element 417, and fed back tothe proportional element 414 and the integral element 415. Theproportional element 414 has a proportionality constant K1 which isexpressed in a unit of sec⁻¹. The integral element 415 has anintegration constant K2 which is expressed in a unit of sec⁻².

The output signal (XX2/J) of the integral element 415 is determined bythe following equation: ##EQU1##

By selecting the constants K1, K2 in order to stabilize the polarity,the above equation is expressed as follows:

    (XX2/J) ≈(X2/J)

    XX2 ≈X2.

Therefore, the disturbance load torque X2 can be estimated by XX2. Theoutput signal from the integral element 415 is representative of anestimated acceleration (XX2/J) which is produced by dividing anestimated disturbance load torque XX2 by J, and is converted into acurrent value by a proportional element 420. For a torquerepresentation, the current value is represented by an estimateddisturbance load torque Ys. J represents the inertia of the spindlemotor 73 as with J in the element 403, and Kt is the same as the torqueconstant of the element 401. A represents a coefficient which is of anumerical value of 1 or less and used to correct the estimatedacceleration (XX2/J). In this manner, the disturbance load torque Ys(X2) of the spindle motor 73 can be estimated using the observer 710.Though the estimated disturbance load torque Ys is of an estimatedvalue, since Ys is described simply as a disturbance load torque in FIG.2, it will be referred to as a disturbance load torque below.

FIG. 1 shows in block form a cutter fracture detecting system accordingto the present invention. As shown in FIG. 1, the cutter fracturedetecting system comprises the observer 710, a filter 2, and acomparator 3.

The observer 710 is one of the functions that are performed by thespindle control circuit 71. In response to a torque command value and avelocity feedback value, the observer 710 calculates a disturbance loadtorque of the spindle motor 73. The calculated disturbance load torquerepresents a torque which is involved purely in a machining process,produced by subtracting acceleration and deceleration torques from alloutput torques of the spindle motor 73.

The filter 2 is a band rejection filter for removing a certain frequencycomponent of the disturbance load torque, and may comprise a twin Tfilter. Since the disturbance load torque generally contains a frequencycomponent that is determined by the rotational velocity of the cutterand the number of cutting edges of the cutter, the filter 2 is used toeliminate the frequency component. If the disturbance load torquecontains two or more frequency components, then a plurality of bandrejection filters each having one of the frequency components at acentral frequency thereof may be connected in series with each other.

FIG. 4 shows frequency characteristics of the band rejection filter. Asshown in FIG. 4, the band rejection filter has a sharp attenuation curveat a frequency which is determined by the rotational velocity of thecutter and the number of cutting edges of the cutter, e.g., at afrequency of 40 Hz.

FIG. 5 shows, by way of example, a filter input signal when the cutteris in a normal condition. When the cutter is in a normal condition withneither of its cutting edges being fractured, the frequency which isdetermined by the rotational velocity of the cutter and the number ofcutting edges of the cutter varies. When all the cutting edges engagethe workpiece equally, the filter input signal is of a signal waveformhaving the same amplitude.

FIG. 6 shows, by way of example, frequency components of a signalinputted to the filter when the cutter is in a normal condition. In theexample shown in FIG. 6, the frequency which is determined by therotational velocity of the cutter and the number of cutting edges of thecutter is 40 Hz.

FIG. 7 shows, by way of example, a filter output signal when the cutteris in a normal condition. In FIG. 7, the signal having the frequency of40 Hz is sufficiently attenuated by the band rejection filter, and thefilter 2 produces substantially no output signal.

FIG. 8 shows, by way of example, a filter input signal when the cutteris in a fractured condition. When one of the cutting edges of the cutteris fractured, a portion of the waveform of an input signal applied tothe filter 2 varies as shown in FIG. 8.

FIG. 9 shows, by way of example, frequency components of a signalinputted to the filter when the cutter is in a fractured condition. Inthe example shown in FIG. 9, several frequency components are generatedaround the frequency of 40 Hz which is determined by the rotationalvelocity of the cutter and the number of cutting edges of the cutter.

FIG. 10 shows, by way of example, a filter output signal when the cutteris in a fractured condition. In FIG. 10, when a signal having frequencycomponents as shown in FIG. 9 passes through the band rejection filterhaving a central frequency of 40 Hz, a varied frequency component is notrejected, but outputted from the filter.

Referring back to FIG. 1, the comparator 3 is connected to the outputterminal of the filter 2. Specifically, one of the input terminals ofthe comparator 3 is connected to the output terminal of the filter 2,and the other input terminal of the comparator 3 is supplied with apredetermined reference value. The reference value may be of a levelLref as shown in FIG. 10.

The comparator 3 monitors the output signal from the filter 2. When afrequency component is varied by a fracture of the cutter, the level ofthe output signal from the filter 2 exceeds the reference value Lref,and the comparator 3 outputs a signal indicative of the fracture of thecutter. The fracture of the cutter can thus be detected.

The signal outputted from the comparator 3 is temporarily stored asbeing latched, for example, and transmitted to the processor 11 in thecomputerized numerical control apparatus 10. Based on the signal fromthe comparator 3, the processor 11 instructs the computerized numericalcontrol apparatus 10 to stop the machining process, issue an alarm, andchange the cutter.

In the above embodiment, the signal outputted from the comparator 3,which indicates a detected cutter fracture, is sent to the computerizednumerical control apparatus 10 for various processes. However, thesignal outputted from the comparator 3 may be used by the programmablemachine controller 16 or an external device connected to thecomputerized numerical control apparatus 10.

With the present invention, as described above, inasmuch as a fractureof a cutter having a plurality of cutting edges is detected based on avaried frequency component of a disturbance load torque, a fracture of acutting edge can be detected easily and reliably. Detection of afracture of a cutter is highly accurate because a disturbance loadtorque is used as a basis for detecting such a fracture of a cutter.

We claim:
 1. A cutter fracture detecting system for detecting a fractureof a cutter which has a plurality of cutting edges, comprising:anobserver to calculate a disturbance load torque of a spindle motor froma torque command value and a velocity feedback value; a band rejectionfilter to remove a given frequency component of said disturbance loadtorque which is determined by a number of the plurality of cutting edgesof the cutter and a rotational speed of the cutter; and a comparator tocompare an output signal from said band rejection filter with apredetermined reference value, and to output a signal indicative of afracture of the cutter when said output signal exceeds saidpredetermined reference value.
 2. A cutter fracture detecting systemaccording to claim 1, wherein said cutter includes a milling cutter. 3.A cutter fracture detecting system according to claim 1, furthercomprising a spindle control circuit to control said spindle motor, saidspindle control circuit including said observer.